1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to zoom lenses of the rear focus type which have as high a range of magnification as 8 and as large an aperture ratio as 1.6-2.4 in F-number with a short total length, and are to be used in photographic cameras, video cameras, or cameras for broadcasting.
2. Description of the Related Art
In the field of zoom lenses for photographic cameras or video cameras, an increasing number of ones in which a lens unit other than the first, when counted from the object side, is moved to perform focusing, or which employ the so-called rear focus type, have been proposed.
In general, the use of the rear focus type decreases the effective diameter of the first lens unit of the zoom lens from that of a zoom lens which performs focusing by moving the first lens unit. This makes it easier to minimize the bulk and size of the entirety of the lens system. Also, close-up photography, particularly supercloseup photography, becomes easy to carry out. Further, because the focusing lens unit is relatively small in size and light in weight, a weak driving power for that lens unit is sufficient. This leads to a possibility of performing rapid focusing.
Of the zoom lenses of the rear focus type having such advantages, for example, Japanese Laid-Open Patent Application No. Sho 63-44614 shows a so-called 4-unit zoom lens comprising, in the order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the magnification, a third lens unit of negative refractive power for compensating for the image shift with zooming, and a fourth lens unit of positive refractive power, wherein the third lens unit is made to move for focusing purposes. However, this zoom lens has to create a space that allows the third lens unit to move and there is a tendency to increase the total length of the entire lens system.
In Japanese Laid-Open Patent Application No. Sho 58-136012, the zooming section is constructed with three or more lens units, of which some are made movable for focusing.
In Japanese Laid-Open Patent Applications No. Sho 62-247316 and No. Sho 62-24213, the zoom lens has four lens units, i.e., a first lens unit, when counted from the object side, of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the image magnification is varied by moving the second lens unit, and compensation for image shift and focusing are performed by moving the fourth lens unit.
In Japanese Laid-Open Patent Application No. Sho 58-160913, there are four lens units, i.e., the first lens unit, when counted from the object side, of positive refractive power, the second lens unit of negative refractive power, the third lens unit of positive refractive power and the fourth lens unit of positive refractive power, the first and second lens units are moved to vary the magnification, while the fourth lens unit is simultaneously moved to compensate for the image shift. Of these lens units, one or two or more lens units are moved to effect focusing.
The related art to the present invention is mentioned in U.S. Pat. No. 5,009,492 and U.S. patent application Ser. No. 534,241 filed on Jun. 7, 1990.
Recently, in the field of video cameras, the trend to minimize the size of the image sensor in the solid-state form (CCD) is advancing. For example, in place of the conventional 1/4 in. or 1/2 in. solid-state image sensing element a smaller size, namely, 1/3 in. or 1/4 in., of image sensing element is being used. In addition, a zoom lens to be used is required to be of a smaller size according to the small-sized image sensing element.
Also, in the photographic lens adapted to be used in the video camera, the distance from the last lens surface to the surface of the image sensing element, i.e., the back focal distance, must be made relatively long. Otherwise, dust or fine foreign particles on the last lens surface would cast their shadow on the image receiving surface, giving bad influence to the image.
However, if, as the design for the zoom lens that is adapted to be used with, for example, the 1/2 in. image sensor, is applied to one which is to be used with the 1/4 in. image sensor, the size of that zoom lens is merely scaled down, the back focal distance gets shorter in proportion (to 1/2). Then, the shadow against the foreign particles or the like on the last lens surface is caused to appear on the image receiving surface, thus giving rise to a problem of lowering the image quality. For this reason, even the reduction of the size of the image sensor leads to a requirement that, as far as the zoom lens for the video camera is concerned, the back focal distance be made longer than a predetermined value.
In general, if the zoom lens of the rear focus type is employed, the bulk and size of the entire lens system is minimized and rapid focusing becomes possible.
On the other hand, however, the range of variation of aberrations with focusing is caused to increase. So, a problem arises in that it becomes very difficult to obtain a high optical performance throughout the entire range of distances of objects from an infinitely distant object to an object at the minimum distance, while still maintaining the minimization of the bulk and size of the entire lens system to be achieved. Particularly, with respect to a large relative aperture, high range zoom lens, the problem of maintaining good stability of high optical performance throughout the entire zooming range and throughout the entire focusing range becomes very serious.